Back to EveryPatent.com
| United States Patent |
6,192,680
|
|
Brugman
, et al.
|
February 27, 2001
|
Subsea hydraulic control system
Abstract
A subsea hydraulic control system 10 supplies hydraulic fluid to operate
subsea equipment SE, which in exemplary application may be a blowout
preventer having an opening port and a closing port. A hydraulic control
system includes a fluid storage vessel 12 for storing a selected quantity
of hydraulic fluid, and a separation member 16 separating subsea water
from hydraulic fluid while pressurizing the hydraulic fluid in response to
the hydrostatic head of the water. A fluid supply line 42 interconnects
the fluid storage vessel and the subsea equipment, and a fluid exhaust
line 44 interconnects the subsea equipment with a subsea hydraulic fluid
reservoir vessel 14. A vent line 38 extends from the fluid reservoir
vessel to the surface. A subsea pump periodically exhausts fluid from the
fluid storage vessel to the water. The method of the invention provides
reliable actuation of subsea equipment, and is particularly well suited
for operating subsea equipment in deep water at depths in excess of 6,000
feet.
| Inventors:
|
Brugman; James D. (Spring, TX);
Elkins; Hubert L. (Kingwood, TX);
Merit; Mark A. (Spring, TX)
|
| Assignee:
|
Varco Shaffer, Inc. (Houston, TX)
|
| Appl. No.:
|
353875 |
| Filed:
|
July 15, 1999 |
| Current U.S. Class: |
60/398; 60/413 |
| Intern'l Class: |
F16D 031/02 |
| Field of Search: |
60/398,413,415,910
|
References Cited
U.S. Patent Documents
| Re30115 | Oct., 1979 | Herd et al. | 251/63.
|
| 3163985 | Jan., 1965 | Bouyoucos.
| |
| 3205969 | Sep., 1965 | Clark.
| |
| 3436914 | Apr., 1969 | Rosfelder.
| |
| 3595012 | Jul., 1971 | Beck.
| |
| 3677001 | Jul., 1972 | Childers et al. | 60/398.
|
| 4095421 | Jun., 1978 | Silcox | 60/398.
|
| 4185652 | Jan., 1980 | Zintz et al.
| |
| 4294284 | Oct., 1981 | Herd.
| |
| 4777800 | Oct., 1988 | Hay, II | 60/593.
|
| 5357999 | Oct., 1994 | Loth et al. | 137/81.
|
Primary Examiner: Ryznic; John E.
Attorney, Agent or Firm: Browning Bushman
Claims
What is claimed is:
1. A system for supplying hydraulic fluid to operate subsea equipment
having first and second fluid input ports, the system comprising:
a subsea hydraulic fluid storage vessel for storing a selected quantity of
hydraulic fluid;
a storage vessel fluid separation member separating water from the
hydraulic fluid in the fluid storage vessel while pressurizing the
hydraulic fluid in response to the pressure of the water at the depth of
the fluid separation member;
an equipment supply line fluidly interconnecting the fluid storage vessel
and both the first and second fluid input ports on the subsea equipment;
a subsea hydraulic fluid reservoir vessel for receiving hydraulic fluid
from the subsea equipment;
a fluid exhaust line fluidly interconnecting the subsea equipment and the
fluid reservoir vessel;
a vent line extending from the fluid reservoir vessel to the surface; and
a subsea pump for exhausting fluid from the fluid reservoir vessel.
2. The system as defined in claim 1, further comprising:
a subsea control system for selectively controlling the flow of hydraulic
fluid from the fluid storage vessel to the first and second fluid input
ports.
3. The system as defined in claim 1, further comprising:
a subsea regulator for regulating the pressure of hydraulic fluid from the
fluid storage vessel to the subsea equipment.
4. The system is defined in claim 3, when the subsea regulator is
responsive to a hydrostatic head of hydraulic fluid in the fluid exhaust
line.
5. The system as defined in claim 1, further comprising:
a selectively actuatable closing valve for controlling the flow of high
pressure fluid to the first fluid input port; and
a selectively actuatable opening valve for controlling the flow of fluid to
the second fluid input port.
6. The system as defined in claim 1, further comprising:
a hydraulic fluid supply line extending from the surface to the fluid
storage vessel for supplying hydraulic fluid to the fluid storage vessel.
7. The system as defined in claim 1, wherein the fluid storage vessel has a
capacity to store at least 80 gallons of hydraulic fluid.
8. The system as defined in claim 1, wherein the pump exhausts the fluid in
the fluid reservoir vessel to the water.
9. The system as defined in claim 1, wherein the pump exhausts the fluid in
the fluid reservoir vessel back to the fluid storage vessel.
10. The system as defined in claim 1, wherein hydraulic fluid is a water
based fluid.
11. A system for supplying hydraulic fluid to operate subsea equipment
having a fluid opening port and a fluid closing port, the system
comprising:
a subsea hydraulic fluid storage vessel for storing at least 80 gallons of
water based hydraulic fluid;
a storage vessel fluid separation member separating water from the
hydraulic fluid in the fluid storage vessel while pressurizing the
hydraulic fluid in response to the pressure of the water at the depth of
the fluid separation member;
a hydraulic fluid supply line extending from the surface to the fluid
storage vessel for supplying hydraulic fluid to the fluid storage vessel;
an equipment supply line fluidly interconnecting the fluid storage vessel
and both the fluid opening port and the fluid closing port on the subsea
equipment;
a subsea hydraulic fluid reservoir vessel for receiving hydraulic fluid
from the subsea equipment;
a fluid exhaust line fluidly interconnecting the subsea equipment and the
fluid reservoir vessel;
a vent line extending from the fluid reservoir vessel to the surface;
a subsea pump for exhausting fluid from the fluid reservoir vessel to the
water; and
a subsea control system for selectively controlling the flow of hydraulic
fluid from the fluid storage vessel to the fluid opening port and the
fluid closing port.
12. The system as defined in claim 11, further comprising:
a selectively actuatable closing valve for controlling the flow of high
pressure fluid to the fluid closing port; and
a selectively actuatable opening valve for controlling the flow of fluid to
the fluid opening port.
13. The system as defined in claim 11, further comprising:
a subsea regulator for regulating the pressure of hydraulic fluid supplied
to the subsea equipment.
14. A method of hydraulically operating subsea equipment having first and
second fluid input ports, the method comprising:
storing a selected quantity of hydraulic fluid in a subsea hydraulic fluid
storage vessel;
separating water from the hydraulic fluid in the fluid storage vessel while
pressurizing the hydraulic fluid in response to the pressure of the water;
fluidly interconnecting the fluid storage vessel and the first and second
input ports of the subsea equipment;
fluidly interconnecting the subsea equipment and a subsea hydraulic fluid
reservoir vessel;
extending a vent line from the fluid reservoir vessel to the surface;
receiving hydraulic fluid from the subsea equipment in the subsea hydraulic
fluid reservoir vessel; and
exhausting fluid from the fluid reservoir vessel.
15. The method as defined in claim 14, wherein the fluid in the fluid
reservoir vessel is exhausted to the water.
16. The method as defined in claim 14, wherein the fluid in the fluid
reservoir vessel is exhausted back to the fluid storage vessel.
17. The method as defined in claim 14, further comprising:
raising the level of the fluid reservoir vessel relative to the level of
the fluid storage vessel to affect the pressure differential to the subsea
equipment.
18. The method as defined in claim 14, further comprising:
selectively controlling the flow of hydraulic fluid from the subsea fluid
storage vessel to the first and second fluid input ports.
19. The method as defined in claim 14, further comprising:
extending a hydraulic fluid supply line from the surface to the fluid
storage vessel for supplying hydraulic fluid to the fluid storage vessel.
20. The method as defined in claim 14, further comprising:
regulating the pressure of hydraulic fluid from the fluid storage vessel to
the subsea equipment.
21. A system for supplying pressurized seawater to operate subsea equipment
having first and second fluid input ports, the system comprising:
the first and the second fluid input ports on the subsea equipment being
selectively in fluid communication with seawater pressurized by its
hydraulic head;
a subsea hydraulic fluid reservoir vessel for receiving seawater from the
subsea equipment;
a fluid exhaust line fluidly interconnecting the subsea equipment and the
fluid reservoir vessel;
a vent line extending from the fluid reservoir vessel to the surface;
a subsea pump for exhausting water from the fluid reservoir vessel; and
a subsea control system for selectively controlling the flow of water fluid
from the subsea equipment to the fluid storage vessel.
22. The system as defined in claim 21, wherein the capacity of the fluid
reservoir vessel is in excess of 100 gallons.
23. A method of hydraulically operating subsea equipment having first and
second fluid input ports, the method comprising:
selectively exposing the first and second input ports of the subsea
equipment to seawater pressurized by its hydrostatic head;
fluidly interconnecting the subsea equipment and a subsea hydraulic fluid
reservoir vessel;
extending a vent line from the fluid reservoir vessel to the surface;
receiving seawater from the subsea equipment in the subsea hydraulic fluid
reservoir vessel; and
exhausting seawater from the fluid reservoir vessel.
24. The method as defined in claim 23, further comprising:
raising the level of the fluid reservoir vessel relative to the level of
the subsea equipment to affect the pressure differential to the subsea
equipment.
Description
FIELD OF THE INVENTION
The present invention relates to a hydraulic control system for operating
subsea equipment. More particularly, this invention relates to a hydraulic
control system for operating subsea equipment at relatively deep water
depths of 6,000 feet or more. The hydraulic control system is capable of
supplying either an opening pressure or a closing pressure to the subsea
equipment.
BACKGROUND OF THE INVENTION
Those skilled in the hydrocarbon recovery industry recognize that an
increasing percentage of hydrocarbons are being recovered from offshore
wells, including wells wherein the subsea wellhead is located in very deep
water of 6,000 feet or more below the ocean surface. Subsea blowout
preventers (BOPs) and related production control equipment rely upon a
source of pressurized fluid to actuate the subsea equipment. Much of this
equipment must be actuatable in at least two directions and thus is
operated by supplying a hydraulic fluid pressure to either "open" or
"close" the equipment. A reliable hydraulic control system to operate the
equipment is particularly important in emergency applications wherein the
equipment must be actuated to either the closed or the opened position in
an emergency.
When subsea equipment is positioned in relatively shallow water of two
thousand or three thousand feet, a reliable pressure source to operate the
subsea equipment commonly is provided by a bank of accumulator bottles
(accumulators), which are conventionally precharged with nitrogen. Each
accumulator is thus a sealed container which houses pressurized nitrogen,
and a bank of such accumulators may be fluidly interconnected to provide
the power source for operating the subsea equipment. The nitrogen thus
acts as an available spring force to operate the subsea equipment once
hydraulic fluid under pressure is pumped into the accumulators from an
external source at the surface. Once the subsea accumulators are
activated, additional hydraulic fluid is conventionally transmitted from
the surface to the subsea accumulators through hose ambilicals or
relatively small conduit fill lines.
While the accumulator system as discussed above performs well on land
operations and in relatively shallow subsea operations, significant
problems are encountered using this accumulator system at water depths of
more than 6,000 feet. The nitrogen precharge pressure must be increased to
overcome the effects of hydrostatic head pressure for water depth of the
control system. Nitrogen, like other gases, has a reduced expansion as the
pressure to which it is subjected gets higher. Moreover, subsea equipment
at 10,000 feet or more is inherently cool, and the combination of the
cooled and high pressure nitrogen approaches saturation so that the
nitrogen tends to lose its expanding characteristics and thus its
pressurizing ability on the subsea equipment. As a consequence, numerous
banks of accumulators are required to reliably supply activating fluid to
a BOP at 10,000 feet, although the same BOP may be reliably controlled at
the surface or in shallow waters with only a few accumulators.
At deep water depths, the hydraulic energy stored in the accumulators may
be at a pressure of several thousand psi in addition to the hydrostatic
head pressure of the surrounding sea water. At a depth of 10,000 feet, for
example, the stored pressure would be approximately 5,000 psi plus 4,450
psi hydrostatic head, for a total of 9,450 psi. At this high pressure, the
nitrogen in the accumulator is very inefficient since flow characteristics
of the nitrogen become very sluggish. For the required capacity of fluid
to reliably operate the subsea equipment, the quantity of accumulators
required is thus significantly increased. This large number of
accumulators represents a high cost to supply fluid to operate the subsea
equipment, thereby increasing the overall cost of the hydrocarbon recovery
operation.
The disadvantages of the prior art are overcome by the present invention,
and an improved hydraulic control system for operating subsea equipment is
hereinafter disclosed.
SUMMARY OF THE INVENTION
A system for supplying hydraulic pressure to operate subsea equipment
utilizes the hydrostatic head pressure of the surrounding seawater to
operate the subsea equipment. A subsea hydraulic fluid storage vessel is
provided for storing a selected quantity of hydraulic fluid, which
preferably is greater than 80 gallons. A piston or other fluid separation
member separates the water from the hydraulic fluid in the fluid storage
vessel and also pressurizes the hydraulic fluid in response to the
pressure of the water at the depth of the fluid separation member, which
in the exemplary application is in excess of 6,000 feet. A fluid supply
line fluidly interconnects the fluid storage vessel and both first and
second fluid input ports on the subsea equipment. A subsea hydraulic fluid
reservoir vessel is provided for receiving hydraulic fluid from the subsea
equipment, and a fluid exhaust line fluidly interconnects the subsea
equipment with this fluid reservoir vessel. A vent line extends from the
fluid reservoir vessel to the surface, which acts as a temporary reservoir
for storing hydraulic fluid discharged from the subsea equipment without
pressurizing the hydraulic fluid by the subsea hydrostatic head. A subsea
pump exhausts fluid from the fluid reservoir vessel. In a preferred
embodiment, the hydraulic fluid is a water based fluid, and the pump
exhausts the fluid from the fluid reservoir vessel directly to the subsea
water.
An electronic subsea control system may be used for selectively controlling
the flow of hydraulic fluid from the fluid storage vessel to the first and
second fluid input ports. A subsea regulator may be positioned along the
fluid supply line to regulate the pressure of hydraulic fluid from the
fluid storage vessel to the subsea equipment. An hydraulic fluid supply
line may extend from the surface to the fluid storage vessel for initially
supplying and resupplying hydraulic fluid to the fluid storage vessel.
According to the method of the invention, a selected quantity of hydraulic
fluid is stored in a subsea hydraulic fluid storage vessel. The hydraulic
fluid is separated from the seawater which automatically pressurizes
hydraulic fluid in response to the hydrostatic head of the seawater
pressure. The fluid storage vessel is fluidly interconnected with first
and second input ports of the subsea equipment and, upon actuating the
equipment, hydraulic fluid is transmitted to a subsea hydraulic fluid
reservoir vessel. A vent line is provided for venting the fluid reservoir
vessel to the surface, and hydraulic fluid in the fluid reservoir vessel
is periodically exhausted, preferably directly to the seawater since the
hydraulic fluid may be water based. In the case of an emergency, the
energy of the hydraulic fluid storage subsea, which is pressurized by the
hydrostatic head of the seawater, is thus available to actuate the subsea
equipment, which may include pipe and shear rams.
It is an object of the present invention to provide a subsea hydraulic
control system which utilizes a hydrostatic head of seawater to provide
the hydraulic energy source which pressurizes fluid to operate the subsea
equipment.
Another object of the invention is to provide a subsea hydraulic control
system which utilizes a subsea hydraulic fluid reservoir vessel, and a
vent line extending from the fluid reservoir vessel to the surface. As a
control system is operated, fluid will be forced by seawater pressure to
the subsea equipment, and fluid from the subsea equipment will be forced
to the fluid storage vessel. As the fluid storage vessel becomes filled
with subsea fluid, a pump may be used to empty the fluid back to the
seawater. A supply line from the surface to the fluid storage vessel may
be used to resupply hydraulic fluid to the fluid storage vessel.
It is a feature of the present invention to provide subsea hydraulic
control system which is well suited for operating in very deep water
depths of 6,000 feet or more.
It is a feature of the present invention that the hydraulic control system
is not adversely affected by the hydrostatic head of seawater at the depth
of the control system, and instead the hydrostatic head is used as the
driving force for energizing the hydraulic fluid. It is a related feature
of the invention that the subsea control system is not adversely affected
by the relatively cool temperatures commonly provided at water depths of
6,000 feet of more.
Another feature of the invention is that the fluid pressure to the subsea
equipment may be easily controlled by a regulator. An electronic control
system may be used to actuate the various valves which control the flow of
fluid from the fluid storage vessel to the subsea equipment and from the
subsea equipment to a fluid reservoir vessel.
Yet another feature of the invention is that seawater pressurized by its
hydrostatic head may be used to operate the subsea equipment.
Still another feature of the invention is that fluid pumped from the subsea
reservoir vessel may be input to the subsea storage vessel, thereby
obviating the use of a hydraulic fluid supply line.
It is a significant advantage of the present invention that the subsea
control system may be used to reliably supply hydraulic fluid pressure to
both open and close subsea equipment at water depths in excess of 6,000
feet and at a cost which is significantly reduced compared to prior art
systems. It is another advantage of the invention that the subsea control
system may be easily redesigned so that seawater rather than hydraulic
fluid may be used to supply the fluid pressure to the subsea equipment,
thereby eliminating the need for the fluid storage vessel and the supply
line from the surface to the fluid storage vessel.
It is another advantage of the present invention that the subsea hydraulic
control system may be reliably used to operate subsea equipment even if
the flow lines which extend from the control system to the surface are
disconnected in an emergency.
These and further objects, features, and advantages of the present
invention will become apparent from the following detailed description,
wherein reference is made to the figures in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a suitable subsea control system
according to the present invention.
FIG. 2 is a schematic illustration of a portion of the control system shown
in FIG. 1 shortly after the hydraulic control system has been actuated to
close the subsea equipment.
FIG. 3 is a schematic view of a portion of the control system as shown in
FIG. 1 with the fluid reservoir vessel positioned at a shallower depth
than the fluid storage vessel.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 is a schematic view of one embodiment of a subsea hydraulic control
system 10 according to the present invention for operating subsea
equipment SE. In an exemplary embodiment discussed below, it will be
assumed that the subsea equipment is a blowout preventer (BOP) located at
a water depth of 10,000 feet. The control system of the present invention
may be reliably used to operate various types of subsea equipment,
including both pipe rams and shear rams, annular BOP's, valves on a subsea
stack, lower riser and choke/kill connectors and podjacks. As shown in
FIG. 1, the exemplary subsea equipment SE is simplistically shown as a
piston P movable within a housing H. The housing includes a closing port
CP and an opening port OP. A rod R is connected to the piston and extends
from the housing. Accordingly, pressurized fluid in the closing port CP
moves the piston to the right from the position as shown in FIG. 1 to the
position as shown in FIG. 2, thereby moving the rod R to a closed
position. Pressurized fluid at the opening port OP moves the piston to the
left from the position as shown in FIG. 2 back to the position as shown in
FIG. 1, thereby moving the rod R to the opened position.
Control system 10 includes a subsea hydraulic fluid storage vessel 12 and a
subsea hydraulic fluid reservoir vessel 14. For the application as shown
in FIG. 1, it may be initially assumed that both the fluid storage vessel
12 and the fluid reservoir vessel 14 are at the same 10,000 foot depth as
the subsea equipment SE. The fluid storage vessel 12 includes a piston or
other fluid separation member 16 which is movable within housing 13. A
conventional seal 18 provides continuous sealing engagement between the
moving piston and the inner wall of the housing 13. As shown in FIG. 1,
the upper end of the housing 13 is open and thus the piston 16 is
subjected to the hydrostatic head of the seawater, which at the 10,000
foot depth is approximately 4450 psi. The piston 16 thus separates the
hydraulic fluid 24 in the fluid storage vessel 12 from the seawater, and
transmits the hydrostatic head of the seawater to pressurize the hydraulic
fluid 24.
Fluid may be initially supplied and is resupplied after actuation of the
subsea equipment SE to the fluid storage vessel 12 via a hydraulic fluid
supply line 28 which extends from the surface to the fluid storage vessel
12. Conventional pump 30 may thus be provided at the surface for pumping
hydraulic fluid at a pressure in excess of 4450 psi through the supply
line 28 to the storage vessel 12. An accumulator schematically shown as 31
located at the surface is thus available for supplying and resupplying
hydraulic fluid to the fluid storage vessel 12. Fluid pressure along the
supply line 28 may be reduced by a conventional pressure regulator 32. A
control valve 34 is shown in FIG. 1 in the closed position, and may be
actuated by an electronic control signal from a MUX electronic control
system described subsequently to be shifted to the opened position for
supplying high pressure fluid to the storage vessel 12. A conventional
check valve 36 may be spaced fluidly between the valve 34 and the vessel
12 to ensure that hydraulic fluid once contained within the vessel 12
cannot flow back to the surface through the supply line 28. The housing 13
must reliably seal the hydraulic fluid 24 from the subsea water, although
the housing 38 normally would not be exposed to a significant pressure
differential since the fluid in the housing is at the same pressure as the
fluid exterior of the housing.
The subsea hydraulic fluid reservoir vessel 14, on the other hand, includes
a housing 15 which must be able to withstand the pressure differential
between the hydrostatic head of the seawater and the relatively low
pressure within the vessel 14. As shown in FIG. 1, the housing 15 is
completely sealed and thus is not exposed to the subsea hydraulic fluid
pressure. A vent line 38 extends from the fluid storage vessel 14 to the
surface, and a check valve 40 may be provided along this vent line to
ensure that fluid flow is limited to the direction from the subsea fluid
reservoir vessel 14 to the surface. The vent line 38 at the surface is
normally open to atmosphere, and accordingly the upper end of the fluid
reservoir vessel 14 is at substantially 0 psi. Another fluid separation
member, such as piston 20, optionally may be provided for fluidly
isolating the air in the upper end of the housing 15 from the hydraulic
fluid 26 in the lower end of the housing 15. A conventional seal 22
provides reliable sealing engagement between the piston 20 and an interior
wall of the housing 15 during movement of the piston. In another
embodiment, the piston 20 may be eliminated.
Hydraulic fluid 24 at 4450 psi is thus continuously available in the fluid
supply line 42 which fluidly interconnects the fluid storage vessel 12 of
the subsea equipment SE. This fluid pressure at 4450 psi may be supplied
to a selectively controlled regulator 48. As explained subsequently, the
depth of the fluid reservoir vessel 14 may be raised above the level of
the fluid storage vessel 12 such that the hydrostatic head of fluid in the
line 46 is also supplied to the regulator 48. For the exemplary
application as shown in FIG. 1, however, the line 46 applies substantially
zero psi to the regulator 48, and accordingly the output from the
regulator may be maintained at any selected value up to a pressure of 4450
psi induced by the hydrostatic head of the water. This high pressure is
thus supplied to the line 50 and the valve 56, and then is input via a
line 54 to the closing port CP of the subsea equipment SE. This high fluid
pressure will thus force the piston P to the right as shown in FIG. 1,
expelling hydraulic fluid through the opening port OP, through the line 55
and through the control valve 58. Hydraulic fluid is thus vented through
the control valve 58 and through line 60, then through the fluid exhaust
line 44 to the fluid reservoir vessel 14.
When it is desired to operate the subsea equipment SE, an electronically
controlled system 80, such as the MUX.TM. control system commercially
available from Varco Shaffer, Inc., may be used to move the control valve
56 to the opened position, as shown in FIG. 1, while maintaining the
control valve 58 in the vent position, as shown in FIG. 1.
Referring now to FIG. 2, once the hydraulic control system has operated the
subsea equipment SE to the closed position, the volume of hydraulic fluid
in the fluid storage vessel 12 is inherently reduced, while the volume of
fluid in the fluid reservoir 14 is increased. Again it should be
emphasized that fluid in the reservoir 14 is not pressurized since the
piston 20 is exposed to substantially atmospheric pressure through the
vent line 38.
Once the hydraulic system has been activated to the position as shown in
FIG. 2, the electronic control system 80 may activate the motor 74 to
power the pump 72, thereby exhausting fluid from the fluid reservoir
vessel 14. The exhausted fluid is thus passed through the check valve 70.
Since the hydraulic fluid is preferably a water-based fluid, fluid may be
exhausted by the pump 72 directly to the water. At the same time, it is
important that fluid be resupplied to the fluid storage vessel 12 so that
a substantial quantity of fluid will again be available to operate the
subsea equipment. Accordingly, the pump 30 may be activated to supply
pressurized fluid through the supply line 28 and to the fluid storage
vessel 12, thereby raising the piston 16 so that a desired quantity of
fluid is again stored in the fluid storage vessel 12. The exhaust pump 72
may discontinue operation once the piston 20 has been lowered sufficiently
so that a quantity of fluid caused by another activation of the system 10
may again be output from the subsea equipment SE to the fluid reservoir
vessel 14.
In an alternate embodiment of the invention, the output from the exhaust
pump 72 may be routed directly to the interior of the fluid storage vessel
12 below the piston 16, rather than expelling the hydraulic fluid to the
seawater. This embodiment thus avoids the requirement of a hydraulic fluid
line to resupply the vessel 12 after actuation of the control system. The
pump 72 is sized for overcoming the hydrostatic head and lift the piston
16 when fluid is exhausted from the vessel 14. A fluid supply line may
thus initially be connected to vessel 12 to supply hydraulic fluid, but
thereafter would only be required to make up for any leakage of hydraulic
fluid from the system.
During a subsequent operation of the hydraulic control system 10, the MUX
system 80 as described herein may actuate the valve 58 to the opened
position, as shown in FIG. 2, and simultaneously shift the valve 56 to the
vent position, as shown in FIG. 2. Pressurized hydraulic fluid is thus
passed by the regulator 48 and through the line 55 to supply pressurized
hydraulic fluid to the opening port OP, thereby moving the piston P back
to the opened position, as shown in FIG. 1. During this opening of the
subsea equipment SE, fluid is thus vented through the closed port CP and
through the valve 56, through lines 62 and 60, through exhaust line 44,
and then to the fluid reservoir vessel 14. Fluid in the vessel 14 may then
be exhausted by actuating the pump 72 as previously described while fluid
is resupplied to the vessel 12 through the supply line 28. Alternatively,
fluid may be exhausted to the vessel 12 via a line which interconnects the
output from the pump 72 with the storage vessel 12, as discussed above.
FIG. 3 discloses a control system as discussed above, except that the fluid
reservoir vessel 14 in this application has been raised above the fluid
storage vessel 12, and in this exemplary application has been positioned
at approximately 6750 feet. At this depth, the hydrostatic head of the
seawater is approximately 3,000 psi, while as previously noted the
hydrostatic head of the water at the 10,000 ft. depth acting on the
hydraulic fluid in fluid storage vessel 12 is approximately 4,450 psi. The
hydrostatic head of fluid in the lower end of the vent line 44 and in line
46 which is input to the regulator 48 is thus approximately 1,450 psi.
Accordingly, for this embodiment the regulator 48 may be set to open at a
differential of 3,000 psi, i.e., the difference between the 4,550 psi and
the 1,450 psi levels input to the regulator 48. As a consequence of
raising the fluid reservoir vessel 14 to this level, the differential
across the piston P in subsea equipment SE is only 3,000 psi, and
similarly the maximum differential across the valve 56 is only 3,000 psi.
By raising the fluid reservoir vessel 14 to 6,740 ft., a back pressure of
1,450 psi is thus always maintained on the piston of the subsea equipment.
Also, this raising of the subsea equipment results in reduced requirements
for the pump 72, which must now only be capable of overcoming the
hydrostatic head of seawater at 3,000 psi when pumping fluid out of the
fluid reservoir vessel 14. By raising the vessel 14, the desired pressure
differential cross the subsea equipment SE may be controlled without the
use of numerous pressure regulators. Also, the wall thickness of the
vessel 14 may be reduced since it seals a lower pressure differential than
the vessel 12.
The method according to the present invention should be understood from the
foregoing description. Subsea hydraulic control system 10 is responsive to
the MUX.TM. Control System, which operates the valves to supply hydraulic
pressure to control the opening and closing (or similar operation) of the
subsea equipment SE. After each sequencing of the equipment SE, pump 72 is
activated to exhaust fluid from the fluid reservoir vessel 14. The valve
34 may also be opened by the MUX.TM. Control System, and the pump 30 at
the surface activated to resupply pressurized fluid to the fluid storage
vessel 12, thereby raising the piston 18 so that a new quantity of fluid
is available to operate the subsea equipment. Alternatively, pump 72 may
discharge fluid from vessel 14 back to vessel 12.
The key to the reliability of the control system according to the present
invention is the ability of the control system to operate the subsea
equipment at least one once during an emergency, i.e., in an offshore
storm or other emergency situation in which the surface vessel must leave
the site. The control system is thus intended to activate or close the
subsea equipment SE after the surface vessel (not shown) is separated from
the supply line 28 and the vent line 38. The selected quantity of fluid in
the fluid storage vessel 12 is thus available to actuate the subsea
equipment, which in an exemplary application consists of closing two sets
of shear rams, closing various valves on the subsea stack, unlocking the
lower riser and choke/kill connectors, and jack-up the pods. Even in this
emergency situation with the line 28 separated, the hydrostatic head of
the seawater pressure acting on the piston 16 thus provides a force which
is required to reliably operate the subsea equipment. Also, even though
the vent line 38 may be separated, the housing 15 is sealed from the
subsea pressure by the check valve 40, and thus substantially only 0 psi
exists within the fluid reservoir vessel 14. If the vent line 38 is
severed in the water, a hydrostatic head of the water may thus act against
the closed check valve 40. The fluid reservoir vessel 14 nevertheless may
be sufficiently sized so that it may receive fluid from the subsea
equipment SE during this emergency situation and yet a sufficiently large
air pocket is maintained within the housing 15 so that the pressure of
hydraulic fluid 26 in the fluid reservoir vessel 14 may rise to, e.g., 100
psi, but will still be substantially lower than the pressure output by the
fluid storage vessel 12. Accordingly, a sufficient pressure differential
will be available to move the piston P to the closed position. The volume
of the vessel 14 may thus be 25% or more greater than the volume of the
vessel 12.
It is thus important that the control system of the present invention
provide the fluid at locations adjacent subsea equipment and stored under
pressure sufficient to operate the subsea equipment. After the emergency
and when the vessel returns to the site and reconnects to the supply line
28 and the vent line 38, the pump 72 may be first activated to exhaust
fluid from the fluid reservoir vessel 14, and the pump 30 then activated
to resupply hydraulic fluid to the fluid storage vessel 12. In another
embodiment of the invention, a battery or other power source may be
provided for operating the motor 74 during an emergency, thereby allowing
the pump 72 to be automatically operated in response to the MUX.TM.
system, thereby ensuring that fluid discharged from the subsea equipment
SE remains at a very low pressure within the fluid reservoir vessel 14.
The pump 72 is sized as explained above, to ensure fluid can be exhausted
from the fluid storage vessel into the water or back to the vessel 12, and
thus is capable of overcoming a hydrostatic head of the water at the depth
of the discharge. The fluid flow capability of the pump 72 need not be
particularly large, however, since the fluid reservoir 14 may be sized to
reliably ensure that it can hold all of the fluid discharged by the subsea
equipment SE. In a typical application, pump 72 may thus be sized for
discharging hydraulic fluid at approximately 25 to 100 GPM into the water.
A pump 30 at the surface must be able to generate a pressure greater than
that necessary to overcome the hydrostatic head of the water at the fluid
storage vessel 12. While it is important to resupply the fluid storage
vessel 12 with hydraulic fluid after the subsea equipment is actuated so
that fluid will again be available for another actuation of the subsea
equipment, the time required to resupply fluid to the vessel 12 is not
particularly critical, and accordingly the pump 30 would typically have a
flow rate of from 25 to 100 GPM. Relatively large capacity pumps are
normally available at the surface to test the subsea equipment via
separate lines which directly connect the surface pumps with the subsea
equipment. Substantially lower capacity pumps may be used, if desired, to
resupply the vessel 12 with hydraulic fluid.
It is important that the fluid storage vessel 12 be sized to make available
to the subsea equipment SE at least the quantity of fluid which will be
required in an emergency to fully actuate the various subsea equipment. In
most applications, the fluid storage vessel SE will be sized to hold at
least 80 gallons of the hydraulic fluid, and typically at least 80 gallons
to 120 gallons of hydraulic fluid. This volume is thus sufficient to
operate subsea equipment as discussed above in an emergency. The fluid
reservoir vessel 14 similarly must have a volume in this range to be able
to hold the fluid exhausted by the subsea equipment SE and, as explained
above, may have a volume in excess of a fluid storage capacity of the
vessel 12 to ensure that, in an emergency, the pressure in the fluid
reservoir vessel 14 remains sufficiently low. The fluid storage vessel may
thus have a capacity in excess of 100 gallons.
Vessels 12 and 14 conventionally may be fabricated from metal, and in a
common application may have a diameter of approximately 20 inches and an
axial length of 8 feet or more. Those skilled in the art will recognize,
however, that the material for the vessels 12 and 14 and the shape of the
vessels are not critical to the present invention, and in alternate
embodiments these vessels may be expandable or flexible bag-like members.
Each vessel may be housed with a portion of the BOP stack guide structure
or a part of the guide structure may form the vessel wall. Also, those
skilled in the art will appreciate that various types of fluid separation
members other than pistons may be reliably used to separate the fluid from
the seawater, and other conventional fluid separation members which
nevertheless reliably transmit hydraulic fluid forces across separation
may be used, such as diaphragms or bladders. Fluid supply line 28 which
extends from the surface to the fluid storage vessel 12 conventionally has
a relatively small diameter, and many applications may be a one inch or
larger line. This flow line must withstand the differential between the
high pressure output by the pump 30 and the hydrostatic head acting on the
flow line at the particular depth under consideration. The flow line
should be sized to achieve relatively low fluid flow losses when fluid is
pumped from the surface to the depths of the fluid storage vessel 12. The
vent line 38 similarly may be a relatively small diameter line and also
must be able to withstand the differential of atmospheric pressure in the
vent line and the high hydrostatic head pressure of water surrounding the
vent line. Both the lines 28 and 38 may conventionally attached to a
marine riser which extends from the surface to the subsea well head.
Those skilled in the art will appreciate that the subsea hydraulic control
system and the method of the present invention are thus well suited for
achieving the objectives discussed above. The system is able to reliably
operate various types of subsea equipment, and the particular subsea
equipment discussed above and simplistically shown in the drawings should
be understood as merely exemplary of conventional equipment which has a
fluid opening port and a fluid closing port, so that the control system is
able to actuate the subsea equipment in both directions, i.e., in an
opening direction and in a closing direction.
In another embodiment of the invention, subsea equipment may be modified so
that it is able to reliably operate in response to fluid which is not
hydraulic fluid and instead is seawater. This may require a substantial
modification to the subsea equipment, since most subsea equipment is
constructed to operate in response to hydraulic fluid and not seawater. A
significant advantage of this alternative embodiment, however, is that the
fluid storage vessel 12 and the supply line 28 may then be completely
eliminated. In one embodiment, the line 42 to the regulator 48 is simply
exposed to seawater. The MUX.TM. system for controlling the valves may
then be used with a fluid reservoir vessel 14. In another embodiment, the
closing port CP and the opening port OP of the subsea equipment SE are
alternatively exposed to seawater. A vent line 38 is nevertheless still
used to ensure that the fluid exhausted from the subsea equipment may be
stored at a pressure substantially less than the hydrostatic head oil the
fluid being input to actuate the subsea equipment.
The control system of the present invention is thus the primary system used
to actuate the subsea equipment during both normal operations and during
an emergency. The system of the present invention is particularly well
suited for operating subsea equipment at relatively deep water depths in
excess of 6,000 feet. While the subsea valves and regulators as described
herein may be reliably operated by a MUX.TM. system of the type available
from Varco Shaffer, Inc., those skilled in the art will appreciate the
various types of control systems may be used to operate the various
components of the subsea system.
Various other modifications may be made to the control system and to the
method of operating subsea equipment may be made without departing from
the spirit of the invention. Such further modifications should be apparent
to those skilled in the art in view of this disclosure. It should thus be
understood that the invention is not limited to the disclosed embodiments
and instead that the scope of the invention should include all embodiments
within the following claims.
Top